Refractive index sensor utilizing gold island surface plasmon resonance on optical fiber

ABSTRACT

A system for measuring an index of refraction that has a light emitting diode and a plurality of reference fibers not in contact with a sample to be measured and that receive light from the light emitting diode. A plurality of sensing fibers with different-shaped plasmon sensors are in contact with the sample and receive light from the light emitting diode. Detectors sense an output of the light from the fibers. The sensing fibers can be arrayed in a planar arrangement, or in a bundle. A cylindrical lens can be used for directing light into the fibers. A plurality of light emitting diodes can be used, each directing its light output into a corresponding fiber. A ball lens can be used for directing the light into the reference fiber. A plurality of wavelength filters can be placed between the light emitting diode and the sensing fiber, and a wavelength of the light entering the fiber may be selected using the filters.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to surface plasmon resonancebased devices. More particularly, the present invention is related tosurface plasmon resonance devices used in a photolithographic system.

2. Related Art

Surface plasmon resonance (SPR) is the oscillation of the plasma of freeelectrons that exists at a metal boundary. These oscillations areaffected by the refractive index of the material adjacent the metalsurface. Surface plasmon resonance may be achieved by using anevanescent wave that is generated when a p-polarized light beam istotally internally reflected at the boundary of a medium, e.g., glass,which has a high dielectric constant. A paper describing the techniquehas been published under the title “Surface plasmon resonance for gasdetection and biosensing” by Lieberg, Nylander and Lundstrom in Sensorsand Actuators, Vol. 4, page 299 (1983).

FIG. 1A shows a diagram of the conventional plasmon sensor equipmentdescribed in the Lieberg paper. An incident beam of light 1 is directedfrom a laser source (not shown) onto an internal portion of surface 2 ofa glass body 3. A detector (not shown) monitors the internally reflectedbeam 4. Applied to the external portion of surface 2 of glass body 3 isa thin film of metal 5, for example gold or silver, and applied to thefilm 5 is a further thin film of material 6. A sample 7 is brought intocontact with the film 6 to thus cause a reaction. If binding occurs, therefractive index of the film 6 will change, and this change can bedetected and measured using surface plasmon resonance techniques.

Surface plasmon resonance can be experimentally observed by varying theangle of the incident beam 1 and monitoring the intensity of theinternally reflected beam 4. At a certain angle of incidence, theparallel component of the light momentum will match with the dispersionfor surface plasmons at the opposite surface 8 of the metal film 5.Provided that the thickness of metal film 5 is chosen correctly, therewill be an electromagnetic coupling between the glass/metal interface atsurface 2 and the metal/sample interface at surface 8, resulting insurface plasmon resonance, and thus an attenuation in the reflected beam4 at that particular angle of incidence. Thus, as the angle of incidenceof incident beam 1 is varied, surface plasmon resonance is observed as asharp dip in the intensity of the internally reflected beam 4 at aparticular angle of incidence. The angle of incidence at which resonanceoccurs is affected by the refractive index of the material against themetal film 5, i.e. the film 6, and the angle of incidence correspondingto resonance is thus relates to the refractive index of the sample.Increased sensitivity can be obtained by choosing an angle of incidencehalf way down the reflectance dip curve where the response issubstantially linear, and then maintaining that angle of incidence fixedand observing changes in the intensity of the reflected beam 4 withtime. This is illustrated in FIG. 1B.

As the angle of incidence is changed, either by moving the light sourceor rotating the glass body, or both, the point on surface 2 at which theincident beam 1 is incident moves. Because of inevitable variations inthe metal film 5 and the film 6, the angle of incidence at whichresonance occurs changes as the point of incidence of incident beam 1moves, which, in turn, introduces a further variable factor into themeasurement and thus makes comparison between the initial unbound stateand the bound state of the film 6 less accurate.

Newly developed lithography machines with immersion have a fluid betweenthe last lens of the projection optics (PO) and the wafer. Ultra purewater is used in such immersion lithography machines that utilizeexcimer lasers (that emit light at a wavelength of, e.g., 193 nm), andflows between the last lens element of the PO and the substrate (e.g., awafer, a flat panel display, a printhead, or the like) in order toenlarge the depth of focus and to enable POs with a numerical aperture(NA) larger than 1. This enables the critical dimension of thesemiconductor devices to be reduced. In order to avoid contamination ofthe projection optics and wafer, the water must be clean. In order toavoid shading effects during projection, the water needs to be free ofparticles and bubbles. Particles are also to be avoided to minimize thenumber of contaminants deposited on the wafer. The supply also needs tomaintain a refractive index (n) of the fluid that is within a very smallrange. However, the refractive index n of the immersion media (IM) canvary due to the introduction of contaminants. Out-of range variations inn will lead to variations in critical dimension and critical dimensionuniformity that will reduce wafer yield.

There is a need in the art for an improved SPR sensor, as well as forapparatus and methods relating thereto.

SUMMARY OF THE INVENTION

The present invention is directed to a refractive index sensor utilizinggold island surface plasmon resonance on an optical fiber thatsubstantially obviates one or more of the problems and disadvantages ofthe related art.

An embodiment of the present invention includes a system for measuringan index of refraction that has a light emitting diode and a referencefiber that is not in contact with a sample to be measured and thatreceives light from the light emitting diode. A plurality of sensingfibers with different-shaped plasmon sensors are also in contact withthe sample and receive light from the light emitting diode. Detectorssense an output of the light from the fibers. The sensing fibers can bearrayed in a planar arrangement, or arrayed in a bundle. A cylindricallens can be used for directing light into the fibers. A plurality oflight emitting diodes can be used, each directing its light output intoa corresponding fiber. Ball lenses can also be used for directing thelight into the fibers. A plurality of wavelength filters can be placedbetween the light emitting diode and the sensing fiber, and a wavelengthof the light entering the fiber may be selected using the filters.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure and particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to illustrate exemplaryembodiments of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1A shows a diagram of conventional plasmon sensor equipment.

FIG. 1B shows a reflectance curve for a conventional plasmon sensor.

FIG. 2 shows a diagram of a lithographic machine where a plasmon sensormay be used.

FIG. 3 shows an exemplary embodiment of a sensor of the presentinvention.

FIG. 4 shows a spectral throughput of a combination of several LEDs andfibers.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 2 illustrates an exemplary lithographic system that utilizes asensor according to the present invention. As shown in FIG. 2, animmersion lithography machine 202 includes an exposure area 204, with awafer stage 206 having a substrate 212 (such as a wafer, or a flat paneldisplay) mounted thereon. Projection optics 214 is used to project lightfrom a light source 222, the light being delivered using a beam deliverysystem 230. An immersion medium (IM) 232 is located between theprojection optics 214 and the wafer 212, and is normally recirculated.An immersion medium supply 216 supplies the immersion medium 213 throughan IM connection 224. Sensors (“MFRIS”) 218, 228 and 208, which arefurther described below, may be located at some or all of the positionsshown in FIG. 2. For example, the sensor 228 samples an IM sample 226,at the location shown. The sensor 218 samples an immersion medium sample220, at the location shown. An additional immersion medium sample 210may be taken at the location shown, for the sensor 208.

Laser light is supplied to the exposure area by the beam delivery system(BDS) 230. The light is sent through the PO 214, the IM 232 and isfinally delivered to the wafer 212. Note that the critical exposure area204 is at the interface of the wafer 212 and the immersion media 232.

The immersion media supply 216 can be inside the immersion lithographymachine 202. A surface plasmon sensor 218 can be incorporated in theimmersion media supply 216, along an immersion media connection line224, or on a wafer stage in the exposure area 204. In either case, asmall portion of the immersion media 232 to be delivered can be sampledby the surface plasmon sensor (208, 218, 228).

Thus, ultra pure water is supplied to an immersion lithography machine202 via an internal ultra pure water supply system 216 that is usually apart of the immersion lithography machine 202. It requires a connectionto the external ultra pure water supply (not shown in the figure) thatis available from the fabrication building that houses the immersionlithography machine 202. The reliance of the ultra pure water supplysystem on an external source of ultra pure water to condition andmonitor the water before delivering it to the exposure area 204 ofmachine 202 requires a set of sensors in the internal ultra pure watersupply 216 to verify index of refraction n. In all such immersionlithography machines 202, the index of refraction n of the immersionmedia 232 must be verified in-situ, to assure proper wafer yield.

Other liquids or gases can be used for the immersion media. Examples ofother immersion media fluids include water with specific salts added toincrease n. Fluids can be used in immersion lithography machines thatoperate with different wavelengths, e.g., 157 nm, 248 nm and 356 nm(either using lasers or using lamp systems forming the light source222). Gases with large values for n can also be used in immersionlithography machines as the immersion media 232.

Conventional sensors either use a simple fiber, or use the same coatingson the all the sample fibers. The same gold island coating on everyfiber leads to the same distribution of island shapes on every fiber.The same distribution of island shapes leads to the resonant absorptionpeak occurring at the same wavelength of the incident light, and thesame red-shift of the peak when a sample liquid or gas is applied theactive areas of the fibers.

In the present invention, instead of utilizing M sample fibers that havebeen prepared together (gold deposition and annealing) in order to formthe same gold island coating on every fiber, the proposed surfaceplasmon sensor has M sample fibers with M different gold island coatings(here, M is an integer).

There are different coatings on at least 2 sample fibers. Also, apreferred embodiment includes M coated reference fibers, each beingprepared with their respective coated sample fiber so that the samecoating is applied to each. There are M pairs of coated sample andreference fibers, with each pair having the same coating applied to bothfibers, but M different coatings are applied to the M pairs. The singleuncoated reference is still required. The same measurement proceduresapply, including that only the M sample fibers are immersed in theliquid or gas to be measured, not the reference fibers.

Applying a slightly different coating to each of the M fibers will leadto M different wavelengths for the absorption peaks. In order to applyslightly different coatings to M fibers, the annealing temperature needsto be carefully controlled. In general, a lower annealing temperaturewill form flatter islands and smaller values of R (R=ratio of theprolate spheroid minor and major axis lengths). The objective is toobtain M different wavelengths for the absorption peaks for the Mfibers. These M different wavelengths are close enough to each other forall of them to be well within the entire range of wavelengths producedby the LED source and the band-pass filters, but are separated enough toallow for easily discernable position of two adjacent absorption peakswith the limited set of wavelengths produced by the LED source and theband-pass filters.

FIG. 3 illustrates one example of a sensor configuration according tothe present invention. As shown in FIG. 3, the sensor configurationincludes a plurality of light emitting diodes (LEDs) 312, and a bandpassfilter 310 for filtering the light from the LEDs 312. A sphericalcollector lens 314 is used to collect the light and direct it to fibers302. Optional ball lenses 308A-308E may be used to direct the light tofibers 302A-302E. Photodiodes 304 (which typically include a matchingphotodiodes 304A-304E for each fiber 302A-302E) are located at theoutput end of the fibers 302, and are then used to measure outputs ofthe fibers 302.

It is to be appreciated that, as discussed below, an exemplary number ofsample and reference fibers are shown, and that increasing the number offibers will increase accuracy and resolution. However, a number ofsample and reference fibers used is application specific.

Also shown in FIG. 3 is a sample area 316, which has an active area 306Aof fiber 302A and an active area 306B of fiber 302B. Note that in thiscase fiber 302A is a sampling fiber, fiber 302B is a reference fiber andfiber 302B is also a sampling fiber. Fibers 302C and 302D are referencefibers, and fiber 302E is an uncoated reference fiber. Also shown inFIG. 3 are the active areas 306 each having a different coating,designated by 320, and including areas 306C and 306D. Separation offibers 302 should preferably be minimized, but must be large enough tonot allow light coupling between fibers 302.

The proposed sensor configuration improves sensor accuracy. The accuracyof the sensor configuration will be increased since the limited set ofwavelengths produced by the LED source 312 and the one or more band-passfilters 310 will now be sampling M different fibers. For some of the Mfibers, the peak may exist at the wavelength produced by a specific LED312 and filter 310 for the others the peak will be between two adjacentwavelengths. A data reduction program can estimate the peak based on theaverage of the M calculated peaks by fitting a curve, such as aparabola, to the set of data points for all available wavelengths. Aparabola can be fitted to each of the M data-sets and the M calibrationparameters provides an n/wavelength shift for each. The effect is thatthe separate calibration parameters improve the resolution (andaccuracy) of the sensor.

Typical fibers that may be used in the present invention are multi-modefibers, with the active area of the sample and reference being in thecentral roughly 1 inch area of the fiber. Typical separation of the FWHM(full width half maximum) of the individual LED and filter combinationsis on the order of 5-10 nm, as shown in FIG. 4.

The gold islands are usually ellipsoidal in shape (i.e., “flattenedfootballs”). The coating and annealing process will lead to a Gaussiandistribution of the ratio R of the minor to major axis of the ellipsoid,centered about a particular center ratio Rc. R can be between 0.1 and0.9 (depends on annealing temperature and time). The use of M fibersentails M different center ratios=RcM. A single island is roughly on theorder of about 100 to 200 nm on the major axis. The number M can be atleast 2, and usually greater than 2. Size of entire sensor increaseswith greater M.

Absorbance of the sample can be calculated as follows:Absorbance sample(As(λ))=−log(‘Sample’ photodiode voltage (Vs(λ))uncoated ‘reference’photodiode voltage (Vr(λ))).Absorbance reference (Ar(λ))=−log(coated ‘reference’ photodiode voltage(Vcr(λ)) uncoated ‘reference’ photodiode voltage (Vr(λ))).Absorbance shift=(lambda where As(λ) is max)−(λwhere Ar(λ) is max).Prior to unknown sample measurement, the sensor measures knownrefractive index liquids to obtain calibration parameter=(n/λshift). TheIndex n of medium=Absorbance shift * calibration parameter.The sensor should preferably be cleaned after every measurement.

Other than the custom coating and annealing of the sample and referencefibers, the components can be off the shelf, there are many commercialsuppliers, such as Thorlabs, Inc. of Newton. N.J., USA, NewportCorporation of Irvine, Calif. USA, or Edmund Optics Inc., of Barrington.N.J.

It will be understood by thosse skilled in the art that various changesin form and details may be made therein without departing from thespirit and scope of the invention as defined in the appended claims.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A system for measuring an index of refraction, comprising: a lightemitting diode; a first reference fiber receiving light from the lightemitting diode and not in contact with a sample; a plurality of sensingfibers each fiber in the plurality of sensing fibers having a plasmonsensor of a different shape, each in contact with the sample, and eachreceiving light from the light emitting diode; additional referencefibers, each fiber in the additional reference fibers having a plasmonsensor of a shape corresponding to a respective one of the sensingfibers, each of the additional reference fibers receiving light from thelight emitting diode, and each of the additional reference fibers not incontact with the sample; a first plurality of detectors sensing outputsfrom the first and additional reference fibers; and a second pluralityof detectors sensing output from the sensing fibers.
 2. The system ofclaim 1, wherein the sensing fibers are arrayed in a planar arrangement.3. The system of claim 1, further comprising a cylindrical lens thatdirects light into at least one of the first reference fiber, one ormore of the plurality of sensing fibers, and one or more of theadditional reference fibers.
 4. The system of claim 1, wherein the lightemitting diode comprises a plurality of light emitting diodes eachdirecting its light output into a corresponding one of the firstreference fiber, the plurality of sensing fibers, and the additionalreference fibers.
 5. The system of claim 1, wherein the plurality ofsensing fibers are arrayed in a bundle.
 6. The system of claim 1,further comprising a ball lens for directing the light into at least oneof the first reference fiber, one or more of the plurality of sensingfibers, and at least one of the additional reference fibers.
 7. Thesystem of claim 1, further comprising a plurality of wavelength filterspositioned between the light emitting diode and the plurality of sensingfibers, wherein a wavelength of the light entering one or more of theplurality of the sensing fibers is selected using the filters.
 8. Amethod, comprising: directing light from an output of a reference fiberonto a detector; directing light from outputs of a plurality of sensingfibers, each having plasmon sensors of different shapes, onto thedetector; directing light from an output of additional reference fibershaving plasmon sensors of shapes that correspond to the sensing fibersonto the detector; and calculating the index of refraction of a sampleof immersion medium in an immersion lithography system based on signalsfrom the detector corresponding to the received light from the referencefiber, the additional reference fibers, and the sensing fibers, whereinthe immersion lithography system utilizes the calculated index ofrefraction during recirculation of the immersion medium.
 9. The methodof claim 8, further comprising selecting a wavelength of light enteringan input of the sensing fibers using wavelength filters.
 10. The methodof claim 8, further comprising focusing light from a light source thatis received at an input of the reference fiber and the additionalreference fibers using a ball lens.
 11. The method of claim 8, furthercomprising focusing light from a light source that is received at aninput of the sensing fibers using corresponding ball lenses.
 12. Themethod of claim 8, further comprising focusing light from a plurality oflight sources into a cylindrical lens, which focuses the light ontoinputs of at least one of the reference fiber, at least one of theadditional reference fibers, and at least one of the plurality ofsensing fibers.
 13. The method of claim 12, further comprising usinglight emitting diodes as the plurality of light sources.
 14. The methodof claim 8, further comprising arranging the plurality of sensing fibersin a bundle.